Catalytic reduction of aromatic mononitro compounds



United States Patent 3,093,685 CATALYTIC REDUCTION OF AROMATIC MONONITRO COMPOUNDS Eugene V. Hort, Plainfield, N.J., Harlan B. Freyermuth, Easton, Pa., and David E. Graham, Westfield, NJ assignors to General Aniline & Film Corporation, New

York, N.Y-, a corporation of Delaware No Drawing. Filed Oct. 21, 1960, Ser. No. 63,968

11 Claims. (Cl. 260-580) This invention relates to an improved method for the reduction of aromatic mononitro compounds, and in particular to an improvement in the catalytic hydrogenation of aromatic monocyclic carbocyclic mononitro'cornpounds to the corresponding amino compounds, and is further particularly concerned with an improved process whereby this reduction is carried out in the liquid phase and in the presence. of the water of reaction as a second discrete liquid phase, no other solvent or diluent being added.

. Aromatic ntiro compounds have long been reduced to the corresponding aromatic amines by a great number of dilferent methods, such as for example by the use of iron borings and dilute acid. In addition, zinc, tin and stannou-s chloride, with or without acid, alkaline sulfides and a great variety of other reducing agents have been used. New techniques have been developed within recent years employing molecular hydrogen with various catalysts to efiect the direct reduction of nitro compounds to the amines. Such catalytic hydrogenation techniques otter many advantages over the previously employed chemical methods, especially with respect to economy, versatility, operating complexities, separation of products, and the ease ofadaptation to continuous processing. Such catalytic hydrogenation techniques have been used with the aromatic nitro compounds in both the liquid and the vapor phase, and a great number of different catalyst systems have been suggested as suitable for the different techniques, primarily because of many inherent disadvantages which come to the front in the use of the catalytic hydrogenation technique. The major problems encountered in the catalytic reduction of aromatic nitro compounds are the fouling of the catalyst or its inactivation, and the production of undesired by-products and side reactions necessitating extremely careful control of operating conditions such as catalyst concentration, catalyst purity, purity of nitro compound, temperatures, pressures, selected solvents and diluents Where employed, and the like. By far, the greatest difiiculties have been encountered in preventing-inactivation of the catalyst by products formed during the reaction such as water, and by the undesired byproducts of the reaction which may result forming undesirable high temperature build-up within the reaction zone in localized areas. Since the reduction of nitro compounds is an exothermic one, it has heretofore been considered essential to operate within careful limits of temperature in order to prevent a running away of the reaction with the possibility of a subse quent violently exothermic decomposition. To obviate such a situation, it has been proposed to employ various solvents and diluents in the reaction zone to moderate the rise of temperature during the reduction stages.

. Among the catalyst systems most widely employed have been those using nickel or other base metals as the active catalytic material. These catalysts usually require rather drastic conditions of temperature and pressure and quite high concentrations of metal to elfect the necessary and desired reduction of nitro compounds to the corresponding amines. One of the major factors in obtaining good yield of amine within reasonable conditions of temperature and pressure is adequate contact between the catalyst, the nitro compound and hydrogen within the reaction zone. Among the suggested. methods for maintaining good contact has been the use of packed towers and other fixed bed catalyst systems wherein the flow liquid nitro compound and hydrogen is so provided as to result in a large contact area between the catalyst and the hydrogen and nitro compound. With the nickel catalyst systems heretofore employed it has been found that the water of reaction tends to cause agglutination of the catalyst and thus retard the speed of the reaction. To overcome this, various techniques have been devised, among them the removal of the Water as vapor formed during the reaction to prevent the formation of a separate aqueous phase, see U.S. Patent 2,292,879. In U.S. Patent 2,458,214, control of the reaction temperature is effected by means of the injection of an inert liquid such as water into the reaction zone and the removal thereof in the form of steam so that there is no separate aqueous phase in the reaction zone.

It has now been discovered that mononitro compounds of the type hereinafter to be described may be catalytically hydrogenated and thereby reduced to the corresponding amino compound in undiluted form at moderate temperatures and pressures employing noble metal catalysts provided the reaction is carried out in a highly stirred reaction zone so that the water formed during the reaction remains Well dispersed throughout the liquid in the reaction zone as a separate and distinct liquid phase. It is preferred that the reaction zone be further provided with some means of cooling to remove the heat of reaction. The process for this invention thereby provides a means for effecting reduction of aromatic mononitro compounds which eliminates the cost of solvents, the contamination of products by solvents, the poisoning of catalysts by solvents, the hazards of volatile inflammable solvents, the poisoning and inactivation of catalysts by undesirable byproducts, the inactivation of the catalyst due to the water formed du-riing their reduction, and finally, the need for elaborate and complex techniques for controlling the temperature of the reaction and the removal of the water formed during the reduction. The yields obtainable by the process of the present invention are substantially quantitative and the products produced thereby are of an excellent quality. It is indeed remarkable and completely unobvious that the hydrogenation can be carried out efficiently, simply and safely by the process of the present invention and that it is not retarded in any Way by the presence of the separate aqueous phase. Not only does this aqueous phase not interfere with the rate of reduction, and as a consequence provides a technique which eifects a reduction at a constant rate, but it appears to offer many advantages by way of temperature control and the lack of production of any undesirable side reactions and reaction products. The present process is not only adapted for batch operation but it is particularly well suited to a continuous reduction technique.

It is therefore an object of the present invention to provide a process for the catalytic hydrogenation of aromatic mononitro compounds.

- It is still another object of the present invention to provide a simple and eflicient process for catalytically hydrogenating aromatic mononitro compounds in the liquid phase.

It is still another object of the present invention to provide a catalytic hydrogenation process for the production of aromatic carbocyclic monocyclic monoamines in the absence of any added solvent or diluent which is safe and efiicient.

It is still a further object of the present invention to provide a process to eflzect the catalytic hydrogenation employing noble metal catalysts of aromatic mononitro monocyclic carbocyclic compounds in the absence of any added solvent or diluent which is safe and efficient and does not require complex techniques for removal of the water of reaction during the course of the reduction.

Other objects will appear hereinafter as the description proceeds.

The aromatic nitro compounds with which this invention is concerned are those characterized as monocyclic carbocyclic mononitro compounds and may be structurally represented by the following formula:

wherein X represents alkyl substituents of from 1 to about 18 carbon atoms and n represents a value of from to 5, inclusive. Specific compounds include the following:

nitrobenzene ortho-nitrotoluene ortho-nitro ethyl benzene para-nitro ethyl benzene 1 the nitroxylenes-e.g., 4-nitro o-xylol, S-nitro m-xylol, 2-

nitro p-xylol nitro(2)-mesitylene -nitro-1,2,3,4-tetramethyl benzene 3-nitro-1,2,4,5-tetramethyl benzene p-nitrotoluene The catalysts which are contemplated herein are the noble metals of the platinum group and include platinum, palladium, ruthenium and rhodium. The catalysts are usually employed on a suitable support in a concentration of from about 0.1% to about by weight based on the Weight of the support and preferably from about 1% to about 5%. Suitable supports are well known in the art and include carbon and alumina. The catalyst system employed should have a surface area of at least about 150 square meters per gram, which characterizes such catalysts as being of the high surface area type. Any of the standard preparations of catalyst may be used. The supported catalysts may be pelleted, granular or powder. They may be on the outside of the support or distributed throughout it. Some of the catalysts which may be employed are exemplified by the following:

Platinum blackSabatier-Reid, Catalysis in Organic Chemistry. D. Van Nostrand Co., New York, 1922.

Platinum oxideAdarns, Voorhees and Shriner, Organic Syntheses, Coll. vol. 1, p. 452. John Wiley & Sons, New York, 1932.

Palladium on charcoalMannich & Thiele, Ber. Deutches pharm. Ges. 26, 36-48 (1916).

Platinum on charcoalEllis, U.S. Patent 1,174,245.

Platinum or palladium on alumina-Schwarcman, US. Patent 1,111,502.

As pointed out above, the present process employs no added solvent or diluent but is operable directly upon the undiluted nitro compounds. The temperature range suitable during the course of the reduction is from about 25 C. to about 125 C. and the pressure employed may range from about atmospheric to about 150 pounds per square inch gage. Increased pressures may be used but since the rate of reaction is not to be increased, no advantage is to be gained thereby.

In order to efiect the desired dispersion of the water formed during the course of the reaction throughout the reaction mass it is necessary to carry out the reaction in the presence of a high degree of suitable agitation. Any means for effecting a well dispersed state of water may be resorted to. In addition to the usual agitation means, one may also employ supersonics in lieu of conventional mechanical agitators. When employing the latter, a peripheral speeding within the liquid of about 500 to 800 feet per minute will produce a suitable and adequate dispersion of the water as a separate and distinct liquid phase.

The following examples will serve to illustrate the present invention without being deemed limitative thereof. Where parts are used, they are to be interpreted as parts by weight unless otherwise indicated.

Example 1 Into an 8 liter reactor there is charged 4500 g. of nitrobenzene and 4.5 g. of a 5% palladium-on-carbon catalyst. The temperature of the reactor is then raised to C. and pressured with hydrogen to 75 p.s.i.g. The reaction mixture is then stirred at 1500 r.p.m. to effect a dispersion throughout the reaction mass of the water as it is formed during the reduction. Hydrogen pressure is maintained at 75 p.s.i.g. by the introduction of hydrogen over a period of 4 hrs. Thereafter the reactants are cooled, discharged from the reactor, and filtered to separate the catalyst. The water of reaction is separated from the oil layer and distillation of the latter yields 3330 g. of aniline with the following analysis:

Percent nitrobenzene (titanous chloride) nil. Purity by bromination Moisture (xylol method) nil. Freezing point 5.8 C.

The yield of aniline is 98%. The catalyst which is recovered is reused and shows virtually the same activity in subsequent runs.

Example 2 The procedure of Example 1 is repeated employing, however, 5000 g. of ortho-nitrotoluene and only 4.0 g. of the same catalyst. A yield of 3860 g. of orthotoluidine (99% yield) is obtained having the following analysis:

Percent ortho-nitrotoluene nil Percent ortho-toluidine (diazo method) 99.6

Example 3 Examples 1 and 2 are repeated employing, in place of 4.5 g. and 4.0 g. of catalyst recited therein, 10 g. of a 5% palladium on alumina catalyst. Comparable results are obtained.

Example 4 Examples 1 and 2 are once again repeated employing, however, 20 g. of the catalyst employed in those examples while maintaining the temperature at 50 C. in place of the 80 C. recited therein. Excellent yields of high purity products are obtained.

Example 5 Example 1 is repeated carrying out the process, however, in a continuous manner using 20 g. of a 5% palladium on carbon catalyst for the charge of 4500 g. of nitrobenzene. The throughput rate is maintained at 4000 g. of nitrobenzene per hour, temperature 80 C. and pressure 50 p.s.i.g. The catalyst is retained in the reactor by means of a microporous metal filter which permits withdrawal of product to the exclusion of the catalyst. A 99% yield of aniline is obtained containing 0.3% of benzene in the efiluent.

Example 6 Example 5 is repeated employing a cascade series of reactors in place of the single reactor of Example 5. Residence time is approximately 1 hr. in the first reactor and in the second and final reactor about 30 min. The yield of aniline is 99+% with no trace of nitrobenzene. In the second reactor there is an initial charge of 4.5 g. of the same catalyst used in the first reactor, and this is similarly retained therein by the metal filter.

Example 7 Example 6 is repeated except that in lieu of microporous metal filters in the reactors, the catalyst is permitted to flow from one reactor to the other and finally is withdrawn with the efiluent and subsequently separated therefrom by centrifuging. The separated catalyst is then recycled with fresh nitrobenzene feed to the first reactor. A small amount of makeup catalyst may also be added to the recycled charge to bring the concentration of catalyst up to the desired level of 20g. per 4500 g. charge of nitrobenzene. This corresponds to a concentration of 0.1% based on the weight of the nitro compound.

Example 8 The procedure of Example 1 is repeated employing as the charge a mixture of ortho and para-nitroethyl benzene. The catalyst is g. of a 5% palladium on alumina and the conditions maintained in the reactor are temperature 50 C. and pressure 50 p.s.i.g. A yield of 97.5% mixed ortho and para-ethyl anilines is obtained.

Example 9 A charge of 4500 g. of mixed nitroxylenes is hydrogenated in the manner of Example 1 using 4.5 g. of 5% platinum on carbon catalyst at 50 C. and 50 p.s.i.g. A yield of 94% mixed xylidines is obtained.

Example 10 Examples 1, 5 and 6 are repeated employing p-nitrotoluene in place of nitrobenzene in these examples. Similar outstanding yields of p-toluidine of excellent purity are obtained.

Other variations in and modifications of the described processes which will be obvious to those skilled in the art can be made in this invention without departing from the scope or spirit thereof.

We claim:

1. A method for reducing aromatic, monocyclic, carbocyclic, mononitro compounds of the formula:

wherein X is alkyl of l to about 18 carbon atoms and n has a value from 0 to 5, inclusive, to the corresponding amino compounds in the absence of added solvent which comprises reducing the said nitro compounds in the liquid phase with hydrogen in the presence of a noble metal catalyst having a surface area of at least about 150 square meters per gram at a temperature of from about 25 C. to about 125 C. and maintaining the water of reaction as a well dispersed separate liquid phase in the reaction zone during the reduction.

2. A continuous method for reducing aromatic, monocyclic, carbocyclic, mononitro compounds of the formula:

wherein X is alkyl of 1 to about 18 carbon atoms and n has a value from 0 to 5, inclusive, in the absence of added solvent which comprises introducing in the liquid phase nitro compound, noble metal catalyst, and hydrogen into a reaction zone wherein the reactants are maintained in a high state of agitation and continuously Withdrawing amino reduction product at a rate equivalent to the feed input rate of the reactants, the temperature of the reaction zone being maintained at about 25 C. to C. and the said noble metal catalyst being characterized as having a surface area of at least about square meters per gram, the water produced during the reaction being maintained as a separate liquid phase in the reaction zone during the reduction and being with drawn from the said reaction zone only as amine reduction product is withdrawn.

3. A method as defined in claim 1 wherein the nitro com-pound is nitrobenzene.

4. A method as defined in claim 1 wherein the nitro compound is a nitrotoluene.

5. A method as defined in claim 1 wherein the nitro compound is a mixture of ortho and para-nitro ethyl benzene.

6. A method as defined in claim 1 wherein the nitro compound is a mixture of nitroxylenes.

7. A method as defined in claim 1 wherein the catalyst is a palladium on carbon catalyst.

8. A method as defined in claim 1 wherein the catalyst is a palladium on alumina.

9. A method as defined in claim 1 wherein the catalyst is a platinum on carbon.

10. A method for reducing nitrobenzene to aniline in the absence of added solvent which comprises reducing the said nitrobenzene in the liquid phase with hydrogen in the presence of a palladium on carbon catalyst having a surface area of at least about 150 square meters per gram, the said reduction being carried out at a temperature of about 80 C. and maintaining the water of reaction as a well dispersed separate liquid phase in the reaction zone during the reduction.

11. A method as defined in claim 10 wherein the catalyst is a 5% palladium on carbon catalyst and the amount employed is about 0.1% by weight of catalyst based on the weight of the nitro compound.

References Cited in the file of this patent UNITED STATES PATENTS 2,619,503 Benner et a1. Nov. 25, 1952 2,772,313 Trager Nov. 27, 1956 FOREIGN PATENTS 832,153 Great Britain Apr. 6, 1960 848,626 Great Britain Sept. 21, 1960 

1. A METHOD FOR REDUCING AROMATIC, MONOCYCLIC, CARBOCYCLIC, MONONITRO COMPOUNDS OF THE FORMULA: 